Polar Bear-Climate Connection Supported by New Study

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May 18, 2009Forecasts of polar bear populations and their likely responses to climate change have been strengthened by a new publication that refutes criticisms of the scientific basis for listing the polar bear under the Endangered Species Act.

The new paper, by a team of scientists from the U.S. Geological Survey (USGS), Woods Hole Oceanographic Institution (WHOI), University of Alaska, University of Maryland, Canadian Wildlife Service and the US Forest Service, refutes point-by-point a widely publicized critique of polar bear population predictions. The new rebuttal reinforces the reports written by the scientists and accepted by the Department of Interior in its May 2008 decision to list polar bears as a threatened species on the U.S. Endangered Species Act.

“The decision to list the polar bear as threatened was politically charged, and the scientific research on which it was based attracted some criticisms. Our new study shows that the critique is incorrect and based on misconceptions about climate models, the Arctic environment, polar bear biology, and statistical and mathematical methods,” said WHOI biologist Hal Caswell, an author on two of the USGS reports and of the rebuttal.

The rebuttal was published in the journal Interfaces online on April 22, 2009, and will be published in the July-August print edition. The journal recently made the article available for free to the public.

In 2007 when the Department of the Interior was considering listing the polar bear under the Endangered Species Act, it asked the USGS to assemble an international team to analyze information on polar bear populations. The team included Hal Caswell, a mathematical ecologist who specializes in developing population models.

Caswell, along with former WHOI postdoctoral investigator Christine Hunter, and researchers from the USGS and other universities and agencies, developed new models that incorporated USGS-collected information about polar bears’ mortality rates, birth rates, life cycles, and habitats. They coupled these models to projections of Arctic climate changes, especially forecasts of sea ice conditions. They calculated the interplay of all these factors – some 10,000 simulations – to estimate the probabilities of future polar bear population growth or decline. Through their study, Caswell,Hunter, and their colleagues were able to link Arctic sea ice directly to population growth.

The USGS-led group presented its reports in fall 2007, and in May 2008, the Department of Interior listed the polar bear as a threatened species under the US Endangered Species Act.

Following that listing, a critique of the USGS reports was published in the Sept.-Oct. 2008 issue of Interfaces, a journal that specializes in management and operations research. Its lead author, Scott Armstrong, a professor of marketing at the University of Pennsylvania, is a key architect of a set of principles on the science of forecasting, which are intended to provide guidance on which methods to use under different circumstances. The principles were derived from such fields as economics, finance, management, politics, medicine, and weather.

In performing its “audit” of the USGS reports, Armstrong’s group applied its set of forecasting principles and claimed that nearly 70 percent of them had been contravened by the USGS reports. The authors of the forecasting audit include a physicist and two economists but do not include biologists, oceanographers or climate scientists.

“We debated writing something short outlining why we don’t think their criticism are valid,” said Caswell. “After going through their report, however, we decided we needed to do a rebuttal of this, and in the end, we went point by point to refute their criticism.”

Caswell continued: “We began by explaining why the sea ice habitat of polar bears is declining and showing how climate models, outputs from which we used as inputs to our analyses, are reliable for forecasting the future climate. Then we showed how each specific criticism of the Armstrong team was either wrong or misleading. Finally, we took a look at their principles of forecasting, and found they are too ambiguous and subjective to be used as a reliable basis for auditing scientific investigations.”

The rebuttal concludes that the audit offers no valid criticism of the USGS conclusion that global warming poses a serious threat to the future welfare of polar bears and that it only serves to distract from reasoned public policy debate.

In the meantime, the USGS continues to collect data in Alaska and Caswell says he will be involved in further analyses of the polar bear populations based on the new data.

The Woods Hole Oceanographic Institution is a private, independent
organization in Falmouth, Mass., dedicated to marine research,
engineering, and higher education. Established in 1930 on a
recommendation from the National Academy of Sciences, its primary
mission is to understand the oceans and their interaction with the
Earth as a whole, and to communicate a basic understanding of the
oceans’ role in the changing global environment.

To feed themselves and their
cubs, polar bears rely on sea ice for platforms to hunt for their main source
of food: seals. Largely because of reduced amounts of sea ice, the polar bear is listed as "threatened" under the Endangered Species Act.
(Chris Linder, Woods Hole Oceanographic Institution)

Mathematical ecologists Hal Caswell of
WHOI (above) and Christine Hunter of the University of Alaska developed
new population dynamics models that documneted for the first time the
critical importance of sea ice for polar bears' survival.
(Tom Kleindinst, Woods Hole Oceanographic Institution)

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I study the mathematical ecology of populations and communities. I use models that include the life cycle, the vital rates, dispersal patterns, and temporal and spatial variation in the environment to explore population growth, extinction, invasion, and spatial pattern, and apply the results to conservation biology.

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